1. Field of the Invention
The invention relates to a semiconductor wafer composed of monocrystalline silicon, and to a method for producing it. The semiconductor wafer has a denuded zone (“DZ”), which is free of BMD defects and extends from a front side of the semiconductor wafer into the bulk of the semiconductor wafer, and a region having BMD defects which extends from the DZ further into the bulk of the semiconductor wafer.
2. Description of the Related Art
Producing a semiconductor wafer involves firstly slicing a substrate wafer from a single crystal pulled according to the Czochralski method. The Czochralski method comprises melting silicon in a crucible composed of quartz glass, dipping a monocrystalline seed crystal into the melt and continued lifting of the seed crystal away from the surface of the melt. In the course of this movement, the single crystal grows at a phase boundary that has formed between the melt and the lower end of the seed crystal during the process of dipping the seed crystal. The substrate wafer sliced from the single crystal is processed to form a polished monocrystalline substrate wafer composed of silicon and is subsequently subjected to a thermal treatment.
By means of this method, a semiconductor wafer composed of monocrystalline silicon (silicon single crystal wafer) finally becomes accessible which has a DZ extending from the front side into the bulk of the semiconductor wafer, which is free of BMD defects (bulk micro defects). In the DZ, defects such as COP defects (crystal originated particles) having a size of more than 20 nm and a density of not less than 2.5×105/cm3 and LPit defects (large etch pit defects) cannot be detected either. The term “front side” usually denotes the surface of a semiconductor wafer which is required for constructing electronic devices. The semiconductor wafer furthermore has a region having BMD defects which extends into the bulk of the semiconductor wafer and which adjoins the DZ.
COP defects and LPit defects belong to defects which arise during the production of a silicon single crystal on account of the supersaturation of intrinsic point defects and are therefore also called grown-in defects. Intrinsic point defects are vacancies and silicon interstitials. COP defects are defects which arise as a result of accumulations of vacancies, and LPit defects are defects which arise as a result of accumulations of silicon interstitials.
BMD defects arise if supersaturated oxygen clusters to form oxidic precipitates. They form centers (gettering sites) which can bind, in particular, metallic impurities. It is therefore desirable for there to be a high density of BMD defects in the bulk of the semiconductor wafer, which permanently keeps such impurities away from the DZ. The presence of free vacancies promotes the formation of nuclei which can give rise to BMD defects.
EP 1 975 990 A1 describes a method by which a semiconductor wafer composed of mono crystalline silicon can be produced, which wafer has a DZ and a region outside the DZ in which the average density of BMD defects is, for example 7×109/cm3. In order to ensure that no grown-in defects are present, in accordance with the teaching of the cited document, during the production of the silicon single crystal, consideration is given to ensuring that the single crystal has an N region extending from the center as far as the circumference of the single crystal.
The “N” region is the designation of a region in the single crystal where the concentrations of vacancies and silicon interstitials are virtually balanced, such that COP defects having a size of more than 20 nm and LPit defects cannot be detected there. The detection of COP defects having a size of more than 20 nm is accomplished, for example, by the scattering of IR laser light at COP defects and the detection of the scattered light (IR laser tomography). One suitable measuring device, which was also used in the examples described here, is the MO-441 from Mitsui, Japan. The IR laser tomography carried out therewith along a fracture edge of the substrate wafer is designated hereinafter as an “MO-441” measurement.
Furthermore, the N region is also free of OSF defects (oxidation induced stacking faults). OSF defects represent dislocation loops. They arise if silicon interstitials are produced on account of oxidation and nucleate at nuclei of oxygen precipitates. An N region in which the number of silicon interstitials exceeds the number of vacancies is called an “Ni region,” and an N region in which this concentration ratio of intrinsic point defects is exactly the opposite is called an “Nv region.”
The ratio V/G of the pulling rate V and the temperature gradient G orthogonally with respect to the phase boundary of the growing single crystal determines as a key parameter whether and to what extent one of the stated types of intrinsic point defects is present to excess in the lattice of the grown single crystal. By control of V/G, therefore, it is possible to produce silicon single crystals which, for example, have over their axial length completely or partly regions in which the N region extends from the center as far as the circumference of the single crystal, or regions which are characterized by a region having COP defects which extends outward from the center and by an adjoining N region extending as far as the circumference of the single crystal.
What can furthermore be achieved by means of the control of the radial profile of V/G is that the N region is completely an Nv region or completely an Ni region or a sequence of radially adjoining Nv and Ni regions.
A semiconductor wafer which is sliced from such a single crystal and processed to form a polished substrate wafer accordingly has from its center as far as its edge an N region or a region having COP defects which extends outward from the center of the substrate wafer, and an adjoining N region extending as far as the edge of the substrate wafer. However, such a substrate wafer still has no DZ and no adjacent region having BMD defects in the bulk of the substrate wafer.
A method also described in EP 1 975 990 A1 uses an RTP (rapid thermal processing) thermal treatment of the substrate wafer for producing the DZ. In this case, the substrate wafer is heated to a temperature of around 1180° C. in a short time, left in this temperature range for a short time and cooled in a short time. The RTP thermal treatment takes place in a gas atmosphere which injects vacancies into the substrate wafer. These vacancies can be eliminated either by recombination with silicon interstitials or by diffusion to the surface of the substrate wafer. The latter possibility can only occur, however, if the diffusion length suffices to reach the surface. One consequence of this mechanism is that an axially inhomogeneous concentration profile of vacancies is present after the thermal treatment, with a depletion of vacancies in a region which adjoins the front side and the rear side of the substrate wafer, if the rear side was also exposed to the vacancy-injecting atmosphere.
The region depleted of vacancies forms the DZ (denuded zone). In the region adjoining it, injected vacancies together with oxygen form complexes which can be nucleated by subsequent thermal treatments and developed into BMD defects.
In accordance with the method described in EP 1 975 990 A1, the substrate wafer is subjected to a post-RTP thermal treatment at a temperature in the range of 800-1000° C. over a period of not more than two hours. The detection of the developed BMD defects comprises a further thermal treatment at a temperature of 900° C. with a treatment duration of 10 hours.
EP 1 887 110 A1 describes the production of a semiconductor wafer composed of monocrystalline silicon from a single crystal which is doped with nitrogen and is pulled in an atmosphere containing hydrogen. The presence of nitrogen in the semiconductor wafer also promotes the formation of nuclei from which BMD defects can arise. Pulling the single crystal in an atmosphere containing hydrogen simplifies the control of the quotient V/G. The range is extended within which the quotient must remain in order that there arises in the single crystal an N region which extends from the center as far as the circumference of the single crystal.
The so-called getter efficiency (“GE”) indicates how effectively BMD defects in the bulk of a semiconductor wafer can keep metallic impurities away from the surface of the semiconductor wafer. If a specific quantity of a metallic impurity such as copper is diffused into the bulk of the semiconductor wafer via the rear side and the quantity of the impurity that can be detected on the front side of the semiconductor wafer is measured, the getter efficiency can be calculated in accordance with the following formula:GE=(1−C/Ct)×100%,where Ct is the quantity diffused into the bulk via the rear side of the semiconductor wafer and C is the quantity of the impurity that is detected on the front side of the semiconductor wafer.
In order to drive the impurity into the bulk of the semiconductor wafer, the semiconductor wafer is usually heated. Kim et al. (Journal of the Electrochemical Society, 155 (11) H912-H917 (2008)) have shown that this drive-in step can take place under specific conditions also at room temperature. In order to drive the impurity out of the bulk of the semiconductor wafer to the front side of the semiconductor wafer, a thermal treatment of the semiconductor wafer can be effected. Such a drive-out thermal treatment that promotes the diffusion of the impurity to the front side of the semiconductor wafer is called low-temperature out-diffusion (LTOD) by Kim et al.
Shabani et al. (J. Electrochem. Soc., Vol. 143, No. 6, June 1996) have shown that copper can outdiffuse from the bulk of a p-doped semiconductor wafer composed of silicon virtually completely to the surface of the semiconductor wafer in a period of a few days even at room temperature if there are not enough BMD defects having a gettering effect in the bulk of the semiconductor wafer.